Microanalytical systems are gaining considerable importance, thanks to their possibility of performing analyses faster and with smaller amounts of material. Their range of application is increasing, together with the progress of diagnostic methods, which rely more and more on molecular biomarkers, but also with the need for on-site monitoring of food quality, environmental contamination, security, forensics, chemical, physical or biological processes. Another field of interest for the microanalytical systems is the development of portable or wearable devices for health monitoring, for persons in difficult environments, e.g. in military, scientific or humanitary operations, for sports, or for dependent persons.
Current microfluidic systems rely on networks of microchannels in which fluids circulate, and can be processed. Typically, said channels are prepared in solid substrates by different methods issued from the electronic industry, such as photolithography, electron-beam lithography, dry or wet etching. They can also be prepared by laser ablation, hot embossing, injection molding, micromachining, or by molding from a previously prepared negative master. Each of these different techniques have their advantages and disadvantages, but also important limitations. A strong limitation for many is the low production rate, and the cost, which make their use in cost-sensitive applications problematic. Injection molding allows for relatively fast and low-cost production, but it is limited to large series, and to a limited number of materials. Also, it requires metal microstructured masters, which are very expensive.
Various attempts have been made to develop solutions to these problems. For instance, WO2006027757 discloses a method for preparing, without expensive fabrication procedures, microfluidic connectors, by polymerizing an elastomer around capillaries in a planar configuration, and removing the capillaries after polymerization.
In Lab Chip, 2012, 12, 2638-3642, Lee et al. proposed a method for preparing complex microfluidic three dimensional networks by preparing connected wires of sucrose, embedding this network in a polymerizable matrix, and then removing the sucrose by dissolution in water. A similar approach, in which the sacrificial material is a metal wire removed by heating, was proposed in Microfluidics and Nanofluidics, 9(2-3), 533-540 (2010) by Song et al. These methods indeed require no sophisticated microfabrication instruments, but they are very labor intensive and require a lot of skills, for the preparation and positioning of the wires.
In the search for cheaper microfluidic devices, attempts have been made to develop microfluidic systems based on paper, popularized as «paper microfluidics». Some systems are reviewed for instance in Anal. Chem., 2010, 82 (1), pp 3-10, Martinez et al. These systems are indeed low cost, and they can be mass produced by printing techniques. They also do not require in general external pumps, since fluid motion is performed by wicking. However, the performances of these systems are limited by the poorly controlled definition of the «channel» walls, by a limited capacity, the importance of evaporation, and the large liquid-solid interface intrinsic to a wicking or capillary mobilization.
In order to improve upon this, various attempts have been made to replace paper by porous textile fibers or «yarn». Some of these attempts are reviewed by A. Nilghaz, et al., in Biomicrofluidics, 7, 051501 (2013): as an advantage, these fibers transport fluids unidimensionally, and this property has been put to work in several aspects. Fluids flow paths are wicking flow paths, consisting in hydrophilic cotton fiber zones delimited by hydrophobic wax patterns.
For instance, US2012192952 discloses a three-dimensional microfluidic system including at least one hydrophilic thread along which fluid can be transported through capillary wicking, and at least one hydrophobic substrate for supporting the hydrophilic thread.
US2011100472 also discloses similar systems, in which exchange of liquid between different yarns is controlled by knots of different topologies.
Along similar lines, US2011189786 to Reches et al. discloses a system for performing an assay on a sample comprising one or more hydrophilic testing threads defining a sample inlet zone at a proximal end, an intermediate zone distal to the inlet zone through which fluid sample is carried by capillary action, and a testing zone distal to the intermediate zone and comprising a detection reagent which reacts with a predetermined analyte to produce a detectable signal. This patent application also discloses a way of separating the threads from the outside world by pressing them between two sheets of plastic impermeable material.
Finally, WO2012150487 and WO 2012/004636 disclose a method of making a diagnostic device, said method comprising: providing at least one strand of a diagnostic-fiber composition; providing at least one strand of a hydrophobic fiber composition; inter-weaving the at least one strand of the diagnostic-fiber composition and the at least one strand of the hydrophobic fiber composition. These methods improve to some extent upon paper microfluidics, but they retain many problems. In particular the necessity of transporting the fluid by wicking along a thread or yarn typically yields strong dispersion. It increases considerably surface to volume ratio, and thus the risk of non specific absorption of contaminants or rare species. Finally, the transport of fluid cannot be easily controlled, and it is restricted to a very limited range of flow rates.
There is thus a strong need for methods for fabrication of microfluidic devices, or more generally minifluidic devices, that would retain the flexibility and low cost of paper or yarn-based microfluidics, but achieve the flexibility and performance of conventional microfluidics operating in open channels.
It was thus an aim of the invention to provide a method to make fluidic devices, and notably minifluidic devices, which is cheap, simple to implement, which provides a device or system comprising at least partly open channels. It was an aim of the invention to provide a method to make a fluidic device with a controlled design of the channel network, and with the possibility to create sophisticated channel designs. It was further an aim of the invention to provide a method to make a fluidic device based on materials of varied chemical nature.